The current plows use single acting hydraulic cylinders to move the blade half from the retracted position to the extended position. The single acting cylinder can only produce force to extend the wing. An extension spring provides opposing force to the hydraulic cylinder to retract the blade.
Since the hydraulic cylinder is single acting, the extension spring is the only means of force preventing the blade half from extending. It is possible to pull the cylinder rod out of the cylinder tube creating a vacuum in the cylinder.
When the plow is being used to back blade (as when pulling snow away from a garage door), only the force from the extension spring resists the snow load and frictional force acting to extend the wings. When the snow load and frictional force exceeds the resistive force of the extension spring, the blades extend forward resulting in loss of the snow mass behind the blade.
An hydraulic means of locking the blade half in place will prevent loss of the snow mass behind the blade.
Double acting cylinders provide an hydraulic means of locking the blade half in place. However, a complicated hydraulic circuit is required. Further, the power unit must produce pressure both to extend and retract the blade half.
The object of the invention is to provide a hydraulic locking cylinder for plow blades to enable back blade plowing.
Locking cylinder embodiments have been developed which will hydraulically lock the blade half in place without the complexity of required in the dual acting cylinder hydraulic circuit. One embodiment uses a counter balance 9 valve located in the piston of the cylinder, and another embodiment uses a pilot operated check valve also in the piston Using a counter balance valve provides a relief function so that when an obstacle is hit during back blading, the cylinder may pass fluid over the relief valve to protect the blade from the impact force. The pilot operated check valve design does not provide a relief function during back blading. By using the locking cylinder design, the complexity of the circuit is reduced and pressure from the power unit is only required when extending the wing. The locking cylinders are interchangeable with existing single-acting cylinders with no additional hydraulics required.